[sdiy] filter response on a scope

Oren Leavitt obl64 at ix.netcom.com
Mon Oct 18 03:45:29 CEST 2010

On 10/17/2010 3:37 PM, Scott Juskiw wrote:
> Use XY mode on your scope. Y is driven by a sawtooth wave (upward ramp) that also controls an oscillator's pitch. Feed the oscillator (sine wave output) into your filter's audio input. X is driven by the output from your filter. Set the filter cutoff frequency as appropriate to get a nice display.

I do this same setup, too - simple and works great!

A linear to log converter on the Y-axis is handy too - this lets you see 
the amplitude in Decibels.
I've used this to put together a good pink noise filter network - get 
that 3dB/Oct drop across the full audio frequency range.

You can also use a voltage-controlled band-pass filter in place of the 
oscillator, and use that setup to view harmonics in a signal fed in to 
the swept BP filter. A poor man's distortion analyzer.

- Oren

> On 2010-10-17, at 2:02 PM, David G. Dixon wrote:
>> Hey Team,
>> Lastnight I designed a board to extract 18 additional filter responses from
>> my 2164 4P LPF, thus bringing the total number of filter responses from the
>> module to 22 (4 LP, 4 HP, 6 BP, 2 N, 4 AP, and 2 more as yet unspecified).
>> It will involve a 1U panel with 18 jacks on it sitting next to the 3U filter
>> panel (hence my last query about PCB mounting), or it could just be a 4U
>> panel.
>> I'd like to be able to look at the filter responses on my old Tektronix
>> scope, but I'm not sure how.  I'm thinking a sawtooth sweep of expo FM on a
>> sine-generating VCO into the filter at constant cutoff, and then perhaps
>> taking the filter output through an envelope follower and then into the
>> scope to see just the positive outline of the response.  However, if anyone
>> has a nice method for doing this, I'd love to hear it.

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